Finish obtainable by milling process depends on insert corner geometry, tool geometry, rake angle, insert tolerance, and operating conditions.
In the case of face milling using an insert with a corner radius r, and using feed per insert (f), the arithmetic average roughness height is proportional to:
Obviously from this formula, for better finish, both ‘r’ and ‘f’ should be as small as possible. However for better surface finish, inserts with corner chamfers are more popular than corner radius inserts. In this case lowest surface roughness height will be possible when the dish angle is zero, i.e., land is exactly parallel to the direction of feed. However to improve the surface integrity, + ve and — ve dish angles are used.
In this case, peak to valley roughness height:
where D = dish angle, f= feed per insert and A = lead angle at cutting point, if dish is + ve
= trailing angle at heel, if dish is – ve
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The most common causes of waviness in face milling are face run-out, spindle run-out, instability of fixtures and clamps, use of a crowned wiper at excessive feed rates.
In case of peripheral milling, the roughness and waviness are dependent upon feed per insert, cutter diameter, number of inserts in cutter body, type of milling (up or down), cutter face runout, spindle runout, stability of fixtures and clamps.
In this case, peak to valley roughness height h = f2/8R where t = feed per insert, and R = cutter radius
Obviously influencing factors are feed per insert, cutter diameter. Up milling produces better finish. It will be seen that if ‘f’ is halved, surface roughness height will be reduced to one-fourth. If cutter diameter is doubled, roughness is reduced to half.
Other factors influencing surface roughness are:
(i) Cutting Speed:
Surface finish is improved as cutting speed is increased. This improvement is more on ductile materials. At low speeds, the shear angle is low and cutting forces are high. Further each section of the workpiece is in contact with the cutting tool for a longer time, thus encouraging built-up edge.
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(ii) Rake Angle:
Surface finish is improved by use of a higher rake angle (more positive or less negative), because it increases the shear angle and thus reduces cutting forces and also the tendency of the workpiece material to adhere to the tool. This effect is more noticeable on ductile materials.
(iii) Dull Insert:
A dull tool has a larger contact area and thus more cutting forces and heat and poor surface finish. Thus before the flank wear reaches maximum value, the insert should be indexed.
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